The invention relates to a method for starting an internal combustion engine for a motor vehicle, said internal combustion engine having at least two cylinders, a valve drive with at least one inlet valve and at least one outlet valve for each of the cylinders, wherein at least the closing times of the inlet valves can be variably adjusted, and a direct injection system, in particular a gasoline direct injection system, the starting process being initiated by a direct injection and ignition in one of the cylinders.
In conventional internal combustion engines, the starting process generally takes place via a so-called pinion starter which accelerates the internal combustion engine by means of a torque applied from the outside up until said internal combustion engine subsequently takes over the further operation by initiating a combustion in the cylinders. The interest in reducing the external torque necessary for starting said internal combustion engine is, for example, also of great interest from usage perspectives. In addition, the pinion starter or, respectively, starter motor can be dimensioned smaller and more cost effectively. The electric machine could be designed smaller in hybrid drive devices because the torque rate for the resistance of the combustion engine drops accordingly.
Such a reduction in torque can be achieved by the use of a so-called decompression start, in which the starting process is initiated by means of injection and ignition in one of the cylinders. In so doing, the first ignition or injection is carried out in the cylinder which is currently situated statically in the expansion phase so that the combustion introduces a torque into the crankshaft of the internal combustion engine. In order to overcome top dead center of the cylinder succeeding in the firing order (succeeding cylinder), it is necessary to discharge a portion of the cylinder charge because the kinetic energy that has built up frequently is not sufficient to compress the complete air mass in the succeeding cylinder. For that reason, a very late closing of the inlet valve is expediently implemented so that the air mass can also be expelled out of the cylinder through the inlet valves. In a six cylinder engine, it can, for example, be necessary to decompress two succeeding cylinders in this manner because a decompression is no longer necessary from the fourth fired cylinder onwards and the full air mass can then be compressed in the succeeding cylinders.